Here's a tiny puzzle game which riffs on the principles of sudoku puzzles. It's called Plutoku and was made by a group of students for a class.
The idea is familiar – you need to fill each row and column with one of each number block (or colour block if you prefer). To add them to the board you click on them as they float past and they will "drop" into the board until they either hit another block or the far side of the board.
The first levels are incredibly easy, particularly if you know that diagonal lines don't conform to the "one of each kind" rule but later levels are more interesting, requiring a degree of logical thought, both in terms of knowing which blocks are needed and adding them in the right order so as not to block access for other blocks.
I do often do sudoku – they're really useful for when I can't sleep – and I think this riff on the basic idea is neat. It's the sort of thing I'd really like to see developed further and made horrifically difficultr.